When a patient is not engaged in a medical treatment, the effectiveness of that treatment is less than the effect of the same medical treatment on a patient that is engaged. Here, the term “engaged” refers to the patient being mentally involved and understanding of the medical treatment.
The placebo effect is where an improvement in medical condition is caused by a patient's belief in a medical treatment as opposed to the actual medical treatment: a patient's positive expectations of perceived treatment are translated by the patient into a positive effect.
There are many placebo effects, with different mechanisms and in different systems, medical conditions, and therapeutic interventions. For example, brain mechanisms of expectation, anxiety, and reward are all involved, as well as a variety of learning phenomena, such as Pavlovian conditioning, and cognitive, and social learning. There is also experimental evidence of different genetic variants in placebo responsiveness. The most productive models to better understand the neurobiology of the placebo effect are pain and Parkinson's disease. In these medical conditions, the involved neural networks have been identified: that is, the opioidergic-cholecystokinergic-dopaminergic modulatory network in pain and part of the basal ganglia circuitry in Parkinson's disease.
Important clinical implications emerge from these recent advances in placebo research. First, as the placebo effect is basically a psychosocial context effect, these data indicate that different social stimuli, such as words and rituals of the therapeutic act, may change the chemistry and circuitry of the patient's brain. Second, the mechanisms that are activated by placebos are the same as those activated by drugs, which suggests cognitive/affective interference with drug action. Third, if prefrontal functioning is impaired, placebo responses are reduced or totally lacking, as occurs in Alzheimer's type dementia.